The $700 million Kepler missionlaunched in March 2009, tasked with determining how common Earth-like planets are throughout the Milky Way galaxy. The spacecraft found alien worlds via the "transit method," noting the tiny dips in brightness caused when planets crossed their host star's faces.

Kepler has discovered 2,682 exoplanets to date, 355 of which were found during the K2 phase. That grand total represents about 70 percent of all known alien worlds. And there will be more Kepler discoveries coming: Nearly 2,900 "candidates" spotted during the original mission and K2 await confirmation by follow-up observations or analysis, and history suggests that most of these will end up being the real deal.

Kepler cannot feasibly be refueled and returned to action. The spacecraft orbits the sun, not Earth, and is currently about 94 million miles (151 million kilometers) from our planet.

Science fiction authors thought about habitable worlds, and even thought about ways that you might try to find them. Consider this idea from Golden age great Edmond Hamilton in his excellent 1936 story Cosmic Quest:

And each mechanical eye, when it found planetary systems in its field, automatically shifted upon them a higher powered telespectroscope which recorded on permanent film the size, mean temperature and atmospheric conditions of these worlds.

(The telespectroscope recorded the conditions of these other worlds)

Doc Smith discussed this matter at length in his 1934 novel Skylark of Valeron:

At the edge of the strange galaxy though they were, many days were required to reduce the intergalactic pace of the vessel to a value at which maneuvering was possible, and many more days passed into time before Crane announced the discovery of a sun which not only possessed a family of planets, but was also within the specified distance of a white dwarf star.

To any Earthly astronomer, whose most powerful optical instruments fail to reveal even the closest star as anything save a dimensionless point of light, such a discovery would have been impossible, but Crane was not working with Earthly instruments. For the fourth-order projector, although utterly useless at the intergalactic distances with which Seaton was principally concerned, was vastly more powerful than any conceivable telescope...

Thus it came about that the search for a planetiferous sun near a white dwarf star was not unduly prolonged, and Skylark Two tore through the empty ether toward it.

(Read more about Manual Search For Habitable Planet)